This invention relates to a locking device for a movement unit that is supported displaceably relative to and along a longitudinal axis of at least one guide rod. The locking device comprises a pair of clamping members comprising each a clamping opening that forms an inner surface for receiving the guide rod. The clamping members have each a foot end and a head end and are pivotable about an axis to assume a first position, in which their inner surfaces are in frictional braking contact with the guide rod, and a second position for allowing free relative movement of the clamping members and the guide rod. To this end, the pair of clamping members has a corresponding pair of control surfaces facing each other at their head ends. To move the clamping members from one position into the other, there is an actuation device which acts onto the pair of clamping members. This actuation device includes a prime mover for imparting motion which comprises a pressure exerting surface for exerting pressure in a direction towards said clamping members. This motion is transferred to a wedge-like actuator separated from the pressure exerting surface to be displaceable transversely to the direction towards the clamping members. The actuator is then pressed by the pressure exerting surface to engage the control surfaces of the clamping members to pivot them into one of their positions.
A vast variety of locking devices has already been suggested in the prior art. For example, DE-A-32 45 074 shows an example wherein lever-like clamping members extend parallel to the guide rod. The actuating device comprises a cam disk to move the clamping levers from one position into the other. However, the orientation of the clamping members parallel to the guide rod (or shaft) to be locked results in a space consuming construction.
U.S. Pat. No. 2,806,723 (FIG. 7) discloses a more compact design wherein a plunger presses a wedge-like actuator which moves the clamping members from one another to assume a respective position. This principle, in an inverted form, i.e. comprising two wedge surfaces which press the clamping members together, can be seen in U.S. Pat. No. 6,044,934. This patent discloses also that the lever-like clamping members should be guided during pivoting from one position into the other by curved guide surfaces in the region of the foot end and the pivoting axis.
This idea has been taken up according to WO 99/38645 wherein the clamping members, thus guided, are able to pivot to a considerable extent. Moreover, this document uses again a wedge member as suggested in the above-mentioned U.S. Pat. No. 2,806,723. However, this wedge is displaceable and floating relative to the pressure exerting surface of the pneumatic plunger along a parallel surface thereof. This allows a more pronounced tilting movement and, thus, better and safer clamping.
Although one was able to achieve a quite compact design by all these measures, the requirements regarding a further reduction of required space, more reliability, longer operation life and so on become higher and higher. To satisfy these requirements is one of the objects of the present invention.
In a surprising simple and very advantageous manner, this object is attained by forming the wedge-like actuator as at least one body of revolution that is able to roll over the pressure exerting surface.
Thus, instead of the usual wedge actuators which slide with friction along the control surfaces of the clamping members engaged by them, a body of revolution is used (i.e. at least one ball or a roller) which can roll over the pressure exerting surface of the actuating device so that at least part of the undesirable friction between the actuator and the clamping members is also avoided. In this way, a series of advantages is achieved, such as:
since the body of revolution does not protrude from a surface parallel to the pressure exerting surface, the locking device is still more compact;
this parallel surface, which up to now stifled the pivoting movement of the clamping members, and the part having this surface are omitted so that one part is saved;
there is more pivoting freedom for the clamping members so that the pivoting stroke (and, thus, the reliability of clamping engagement) is increased, particularly as the center (in longitudinal direction) remains undefined;
there is less wear due to rolling friction rather than the known sliding friction, thus increasing the reliability and operation life;
assembly is made easier, because the body of revolution orients itself when engaging the control surfaces of the clamping members;
Since the body of revolution has only punctual contact (in case of a ball) or line contact (in case of a roller which is preferred) with the pressure exerting surface, no tilting forces act onto the latter surface, i.e., in the case of a plunger with such a surface, that the plunger can move linearly within its cylinder without any risk of jamming;
this, in turn, means that the plunger itself can be shorter (in the direction of its movement), i.e. it needs less surface to be linearly guided in the cylinder which favorizes further a compact design.
In the case of a roller as a body of revolution, it could have different diameters over its length to be, for example, smaller in the middle (or larger) than at its ends, the control surfaces of the clamping members preferably corresponding to this variation in diameter and are, for example convex. However, a cylindrical roller is preferred.
Since the pivoting stroke is larger according to the invention, it is of special benefit if the locking device comprises those characteristics which are described in U.S. Pat. No. 6,044,934.
Since the clamping members are enabled, according to the invention, to pivot over a larger stroke than according to the prior art, it is useful to form the inner surface of at least one clamping member with a substantially convex cross-section when seen in the direction of longitudinal axis of the guide rod. xe2x80x9cSubstantially convexxe2x80x9d does not mean that it must be uniformly curved, but means that it could also be prismatic or cornered. In this way, a relative large area of the inner surface will engage the rod (or shaft) and will ensure a large locking force.
Since a body of revolution has a generally wedge-like shape, i.e. its width increases up to its middle, the clamping members could, in principle, be completely straight so as to extend in the direction of the actuation movement. However, it is preferred if the control surface is formed as a tread surface for said body of revolution under an angle to the direction of movement of the actuating device. In this way, the additional advantage is achieved that a special return motion device for the actuating device, as had been provided in the prior art either as a spring seated between the clamping members or a spring which acts onto the actuating device itself, is no longer necessary. This results in a simplification of the design and a cost saving production.
If the body of revolution is dimensioned so as to penetrate in each of the positions of said clamping members deeper between their control surfaces than it projects over their head ends towards said pressure exerting surface, a kind of cage bearing for the body of revolution is achieved which ensures its position between the clamping members.
The movement imparting means can be of any type, e.g. formed by an electromagnet. Preferably, however, it comprises a fluidically actuable plunger.